ANDROID: fixup ABI break in struct readahead_control.

Add new field "dropbehind" in a hole without breaking ABI in "struct readahead_control".

before:

    struct readahead_control {
            struct file *              file;                 /*     0     8 */
            struct address_space *     mapping;              /*     8     8 */
            struct file_ra_state *     ra;                   /*    16     8 */
            unsigned long              _index;               /*    24     8 */
            unsigned int               _nr_pages;            /*    32     4 */
            unsigned int               _batch_count;         /*    36     4 */
            bool                       _workingset;          /*    40     1 */

            /* XXX 7 bytes hole, try to pack */

            unsigned long              _pflags;              /*    48     8 */
            u64                        android_oem_data1;    /*    56     8 */

            /* size: 64, cachelines: 1, members: 9 */
            /* sum members: 57, holes: 1, sum holes: 7 */
    };

after:

struct readahead_control {
	struct file *              file;                 /*     0     8 */
	struct address_space *     mapping;              /*     8     8 */
	struct file_ra_state *     ra;                   /*    16     8 */
	unsigned long              _index;               /*    24     8 */
	unsigned int               _nr_pages;            /*    32     4 */
	unsigned int               _batch_count;         /*    36     4 */
	bool                       _workingset;          /*    40     1 */
	union {
		bool               dropbehind;           /*     0     1 */
		unsigned char      __kabi_ignored1;      /*     0     1 */
	};                                               /*    41     1 */

	/* XXX 6 bytes hole, try to pack */
	union {
		bool                       dropbehind;           /*     0     1 */
		unsigned char              __kabi_ignored1;      /*     0     1 */
	};

	unsigned long              _pflags;              /*    48     8 */
	u64                        android_oem_data1;    /*    56     8 */

	/* size: 64, cachelines: 1, members: 10 */
	/* sum members: 58, holes: 1, sum holes: 6 */
};

Bug: 416315033
Change-Id: I6f47006ecef760e988e0485e0648c54ad2931644
Signed-off-by: Akilesh Kailash <akailash@google.com>