vfs: separate FMODE_PREAD/FMODE_PWRITE into separate flags

Separate FMODE_PREAD and FMODE_PWRITE into separate flags to reflect the
reality that the read and write paths may have independent restrictions.

A git grep verifies that these flags are always cleared together so this
new behavior will only apply to interfaces that change to clear flags
individually.

This is required for "seq_file: properly cope with pread", a post-2.6.25
regression fix.

[akpm@linux-foundation.org: add comment]
Signed-off-by: Paul Turner <pjt@google.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc:  Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: <stable@kernel.org>		[2.6.25.x, 2.6.26.x, 2.6.27.x, 2.6.28.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>