fs/reiserfs: atomically read inode size

See i_size_read() comments in include/linux/fs.h

Link: http://lkml.kernel.org/r/20170123174701.30394-1-fabf@skynet.be


Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>