libext2fs: add sanity checks for ea_in_inode

libext2fs: add sanity checks for ea_in_inode

An inode containing the value for an extended attribute (aka an
ea_in_inode) must not have the INLINE_DATA flag and must have the
EA_INODE flag set.  Enforcing this prevents e2fsck and debugfs crashes
caused by a maliciously crafted file system containing an inode which
has both the EA_INODE and INLINE_DATA flags set, and where that inode
has an extended attribute whose e_value_inum points to itself.

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>