btrfs: use btrfs_inode local variable at btrfs_page_mkwrite()

Most of the time we want to use the btrfs_inode, so change the local inode
variable to be a btrfs_inode instead of a VFS inode, reducing verbosity
by eliminating a lot of BTRFS_I() calls.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>