f2fs: Remove F2FS_P_SB()

All callers have been converted to F2FS_F_SB() so delete this wrapper.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>