simple_xattr: permit 0-size extended attributes

simple_xattr: permit 0-size extended attributes

If a filesystem uses simple_xattr to support user extended attributes,
LTP setxattr01 and xfstests generic/062 fail with "Cannot allocate
memory": simple_xattr_alloc()'s wrap-around test mistakenly excludes
values of zero size.  Fix that off-by-one (but apparently no filesystem
needs them yet).

Signed-off-by: Hugh Dickins <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Aristeu Rozanski <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>