ext4: add bounds checking to ext4_xattr_find_entry()

Add some paranoia checks to make sure we don't stray beyond the end of
the valid memory region containing ext4 xattr entries while we are
scanning for a match.

Also rename the function to xattr_find_entry() since it is static and
thus only used in fs/ext4/xattr.c

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org